Orgasm should heal the character in some way, and consume some renewable resource (magicka, stamina, ...), it should also consume some arousal. Repeated orgasms quickly become exhausting and gameplay should reflect that.

Arousal should ideally tie in to conversation mechanics.  (Very low arousal should make sexual activities seem repellent. Very high arousal should make non-sexual activities seem repellent.) But conversation mechanics are complicated.

Combat, of course, takes priority when it happens...

Mods already support orgasm mechanics, so this would probably be plugged into existing mods...

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NPCs

Generally speaking, npcs should have a priority queue of goals they work towards, with lower priority goals only kicking in when the current goal doesn't provide a relevant decision.  Choices should each have a success estimate on them, and when choices get used they should be marginalized (either reduce that success estimate for that specific context, or throw them to the bottom of a priority queue or some such, to avoid "running in circles, indecisively").

Time consuming goals (which should reflect the NPC's normal routine) should ideally have animations associated with them. Story goals should have conversation trees (and the NPC should have some simplified model of things - succeeding and failing should get reactions -- and ideally sexual context should get reactions). Sex goals can be handled with a fade-to-black or with text when there's no approximate animations for them.

NPCs are going to tend to have excuses for their non-participation in player activities (except for followers, when following). A lot of this would be implied, don't overdo it...

Anyways, constant sex imposed by NPCs would drag the game to a halt, but a complete absence of sex makes sex mods seem silly. Pacing mechanisms based on player character arousal and on NPC arousal and on game events / locations / structure can deal with that, but tuning it all takes work (probably the player should have a config option to partially control the pacing and spacing of events).

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Mode of dress or other gearing issues can telegraph a lot, but people mostly notice when things change. So it's kind of important to switch things up.